Conventionally, downhole pumps are actuated using a rod pump, commonly known as a pump jack, which reciprocates a rod string operatively connected to the pump. Alternatively, a rotary pump such as a PC pump can be actuated by rotating the rod string using a rotary drive system at surface.
Stuffing boxes are commonly positioned on wellheads having such pumps to create a seal between the wellhead and rod string, or other wellbore tubulars, extending therethrough in order to retain fluid pressures therebelow and prevent the leakage of wellbore fluids past the stuffing box to the environment.
Typically, the stuffing box is positioned to secure around the uppermost rod of a plurality of connected rods forming the rod string, referred to as the polished rod. In order to create a seal with the polished rod and allow the rod to reciprocate through, or rotate in, the stuffing box without being damaged, one or more seals, such as packing rings, are positioned within the stuffing box and concentrically disposed around the rod. Such designs are successful when the polished rod is axially aligned and concentric with the bore of the stuffing box. However, for various reasons, the polished rod can be out of alignment with the stuffing box bore and apply a side-load to the seals therein, consequently causing excessive wear to the packing rings and/or the rod with a resultant leak thereby. For example, the polished rod can become misaligned due to settling of the pumpjack or rotary drive on a base, or misalignment of the wellhead and/or stuffing box during assembly. In extreme cases of wear, the polished rod can weaken and fail and the remaining rod string can drop into the wellbore, leaving the wellbore dangerously open to the surface. Further, retrieval of a dropped rod string from the wellbore can be time consuming and costly.
Often, stuffing boxes provide a safety valve for closing off the wellbore from surface in the catastrophic event of breakage of the polished rod. Known valves typically comprise a movable portion such as a flapper that is pivotally mounted in the bore of the stuffing box and biased to pivot from an “open” position, which allows the polished rod to extend there through, to a “closed” position, which effectively seals the surface off from wellbore fluids. Such safety valves are also vulnerable to damage by a misaligned rod, thus introducing the risk of a damaged valve failing to prevent wellbore fluids from escaping to surface in the event of rod failure.
To mitigate damage to stuffing box components caused by a misaligned polished rod, it is known to provide an adjustable stuffing box capable of aligning with a misaligned rod. For example, there exist self-aligning stuffing boxes incorporating two component stuffing boxes with an angular alignment interface. Such units have a first tubular member having a convex face for engaging a concave seat on a second tubular member, forming an alignment aspect therebetween and permitting limited universal angular movement of the first and second members relative to one another for aligning the packing seals with the polished rod therein. Such self-aligning stuffing boxes typically necessitate a relatively tall body to accommodate the alignment aspect, and can only withstand relatively low wellbore pressures compared to non-self-aligning stuffing boxes.
Other designs for adjustable stuffing boxes, such as that described in U.S. Pat. No. 5,711,533 to Angelo et al, employ a flanged connection between an upper sealing section and a lower wellhead mounting section, such flanged connection having oversized bolt holes and oversized washers to allow the upper section to be displaced laterally relative to the lower section so as to align with the polished rod and alleviate side loading on the stuffing box seals therein. While such stuffing boxes provide reduced height and better high pressure performance due to the otherwise flanged interface, they can be problematic as the range of lateral adjustment of such boxes is limited by the size of the bolt holes. An increase in the size of bolt holes necessitates an increase in flange size, washers, and/or bolt heads to accommodate the larger bolt holes, perhaps requiring non-traditional washers or bolts which may be more expensive and/or difficult to replace. Eventually, too great a disparity between bolt and hole diameters may adversely affect the stability of the stuffing box assembly.
Additionally, many existing adjustable stuffing boxes merely rely on the stuffing box seals to forcibly align the polished rod with the stuffing box, which puts significant side loads on the seals and can cause premature wear thereto.
Further, existing procedures for aligning an adjustable stuffing box with a polished rod involve approximating the position of the stuffing box where it might be best aligned with the polished rod. Such an alignment procedure is inaccurate and may result in the unwanted side-loads.
There is a need for an adjustable stuffing box capable of self-alignment with a misaligned polished rod that has a low profile, is capable of withstanding high pressures, can be readily aligned without subjecting the stuffing box seals to excessive side loads, and can be used with commonly available components. Further, it would be ideal to accommodate or equip the stuffing box with a safety valve for sealing the wellbore in the event catastrophic failure of the rod occurs.